Conventionally known is an EGR device for recirculation of a part of exhaust gas from, for example, a vehicle engine to the engine to suppress generation of nitrogen oxides. Some EGR devices are equipped with, midway of an exhaust gas recirculation line to the engine, an EGR cooler for cooling of the exhaust gas since cooling the exhaust gas to be recirculated to the engine will drop a temperature of and reduce a volume of the exhaust gas to lower a combustion temperature in the engine without substantial decrease in output of the engine, thereby effectively suppressing generation of nitrogen oxides.
FIG. 1 is a sectional view showing an example of the EGR cooler in which reference numeral 1 denotes a cylindrical shell with axially opposite ends to which plates 2 are respectively fixed to close the ends of the shell 1. Penetratingly fixed to the respective plates 2 are opposite ends of a number of tubes 3 extending axially in the shell 1.
Cooling water inlet and outlet pipes 4 and 5A are attached from outside to the shell 1 near one and the other ends of the shell 1, respectively, so that cooling water 9 is fed through the inlet pipe 4 into the shell 1, flows outside of the tubes 3 and is discharged outside of the shell 1 through the outlet pipe 5.
The respective plates 2 have, on their sides away from the shell 1, bowl-shaped hoods 6 fixed to the plates 2 so as to enclose end surfaces of the plates 2. The one and the other hoods 6 provide central exhaust-gas inlet and outlet 7 and 8, respectively, so that exhaust gas 10 from the engine enters through the inlet 7 into the one hood 6, is cooled during passage through the number of tubes 3 by heat exchange with cooling water 9 flowing outside of the tubes 3 and is discharged into the other hood 6 and recirculated through the outlet 8 to the engine.
In the figure, reference numeral 11 denotes a bypass outlet pipe, arranged at a position diametrically opposed to the cooling water inlet pipe 4, through which a part of the cooling water 9 is withdrawn to prevent the cooling water 9 from stagnating at the position diametrically opposed to the cooling water inlet pipe 4.
Such conventional EGR cooler has poor heat exchange efficiency since the exhaust gas 10 may flow straight in the tubes 3 and insufficiently contact inner peripheries of the tubes 3. Thus, it has been proposed that inner peripheries of the tubes 3 are formed with spiral protrusions 12 (the tubes 3 are concaved into spiral grooves on outer peripheries thereof to thereby provide the spiral protrusions 12 as inverse formations on the inner peripheries) to causes the exhaust gas 10 flowing through the tubes 3 to whirl, thereby increasing contact frequency and contact distance of the exhaust gas 10 to the inner peripheries of the tubes 3 to enhance the heat exchange efficiency of the EGR cooler (see, for example, Patent Literatures 1 and 2).
As prior art literatures pertinent to the invention, there already exist, for example, the following Patent Literatures 1 and 2.